1. Unit 9-1: The Rock Cycle Part 1

2. The rock cycle begins as magma cools and hardens into rock
The rock cycle begins as magma cools and hardens into rock. This type of rock is known as Igneous Rock.

3. Extrusive Igneous Rock: Forms at surface when a volcano erupts.
So there’s basically two types of igneous rock, depending on where the magma cooled!
Intrusive Igneous Rock:
Forms when magma cools
below the surface of the Earth.

4. Two types of lava/magma
Mafic Magma: -Low silica content, but high calcium, iron and magnesium.
-Hotter and more fluid than Felsic.
-Forms darker minerals and rocks when it cools.
-Ex. Horneblende, Augite, Basalt.
-Mostly makes up extrusive igneous rocks.
Felsic Magma: -High silica content.
-Thick and slow flowing.
-Forms light colored minerals and rocks.
-Ex. Quartz, Orthoclase Feldspar, Granite.
-Mostly makes up intrusive igneous rocks.

5. So what you’re telling me is that a rock’s physical properties are
Igneous rocks can be glassy-smooth in texture, or they can be rough and coarse in texture.
So what you’re telling me is that
a rock’s physical properties are
dependent on its composition,
AND how/where it cooled?
The texture of the rocks depends on the
size, shape, and arrangement of its grains
or crystals (from the minerals that make
up the rock.)
Another factor is the percentage of dissolved gas in the magma. Gas keeps the magma hotter for longer.
Some igneous rocks, like obsidian, contain grains that are too small to be seen by the naked eye.
Another factor is the order in which the crystals form. This is because different rocks melt/solidify at different temperatures.
The size of grains is dependent on how fast the magma that formed the rock cooled. The faster the magma/lava cools, the smaller the crystals.

6. -Some igneous rocks have two distinct textures.
-When large crystals are surrounded by a fine grained or glass rock (called a groundmass), the rock is a porphyry.
Now this rock has fine mineral
grains and large mineral grains.
So how do we figure out where
it formed?
-A porphyry forms in two stages.
-First, it begins to form deep in the earth, where it cools slowly.
-This allows the large crystals to form.
-Then the magma slowly rises to the surface.
-Once it breaks to the surface, it cools very quickly.
-This forms the groundmass of very small crystals.

7. -Now there are three families of igneous rocks, based on
their composition. Each family has members based on
the grain size.
-The families are the granite family, the gabbro family, and
the diorite family.
-There is a fourth igneous rock type called granodiorite.
Now, let’s take a look at these individually.

8. The Granite Family -Made of high-silica magmas.
-Made mainly of orthoclase feldspar and quartz.
-There can also be plagioclase feldspar, mica, and horneblende.
-Rocks in this family are usually light in color.
Based on grain size:
-Granite is coarse-grained.
-Rhyolite is fine-grained.
-Obsidian and Pumice are glassy.
-All three have the same chemical structure.

9. The Gabbro Family -Rocks in this family have low silica content.
-Made mainly of plagioclase
feldspar and augite.
-Can also contain olivine,
horneblende, and biotite.
-All minerals in this family
(except feldspar) contain
high levels of iron.
-These rocks are usually
dark in color.
-In this family, gabbro is coarse-grained.
-Basalt is fine-grained.
-Basalt glass is glassy.
-Diabase is somewhere between basalt and gabbro in texture.

10. The Diorite Family
-Rocks in the diorite family have a texture that is somewhere in between the granite and gabbro family.
-They are made of plagioclase feldspar, horneblende, augite and biotite.
-They are a mix of dark and light colors.
-Diorite is the coarse-grained rock in this family.
-Andesite is the fine-grained rock in this family.
-Obsidian without quartz in it is the glassy rock in this family.

11. Granodiorite
-Between the coarse textured rocks in the granite and diorite family is granodiorite.
-It shares in compositions somewhere in between granite and diorite.
Three other coarse grained rocks are of interest as well:
-Pyroxenite is almost entirely pyroxene.
-Dunite is almost all olivine.
-Periodotite is a mixture of olivine and pyroxene.

12. I think that if rocks formed at different locations and have different compositions, then they should have different properties.
Hey, now that we’ve looked at how igneous rocks form and their classifications, can we look at some of the properties of the individual rocks themselves?

13. -The quartz looks like gray or smoky glass.
-Granite is made from quartz, orthoclase feldspar, and at least one other mineral, such as mica or horneblende.
-The quartz looks like gray or smoky glass.
-The feldspar is usually white, gray, or pink, and is noticeable by its flat fracture planes.
-The mica can be easily broken away, while the horneblende cannot.
-Granite can come in many different colors.
-Granite is the most common continental igneous rock.
-It can be found in the Rockies, the Adirondacks, the Black Hills, etc.
-Granite is an intrusive rock, so it is only exposed to the surface after a great amount of erosion.

14. -Obsidian is formed by very rapid cooling of lava.
-It is dark brown to black in color, and is glassy in texture.
-Obsidian is a very hard rock, but also very brittle.
-When broken, it has very sharp edges.
-Pumice is made by very rapid cooling of lava, when dissolved gasses cannot escape.
-It looks like a sponge, but this rock is glass, like obsidian.
-Because of the trapped gas, pumice is light enough to float.

15. -Basalt is the igneous rock of the ocean floor.
-This is the type of rock that forms from surface lava flows that cool moderately slowly.
-Fine grained, colors range from black to dark green.
-Diabase is similar to basalt.
-Made from the same material as basalt, but with larger grains.
-Also known as dolerite or trap rock.